Moving coil magnetic pen recorder with coupled driving and velocity coils and a mutual inductance compensating bucking signal coil



1966 A. D. BROWN, JR. ETAL 3,281,638

ORDER WITH COUPLED DRIVING AND VELOCITY COILS AND A MUTUAL INDUCTANCE MOVING COIL MAGNETIC PEN REC COMPENSATING BUGKING SIGNAL COIL 5 Sheets-Sheet 1 Original Filed Sept. 5. 1961 Wm a. fl W Oct. 25, 1966 A. D. BROWN, JR. ETAL 3,281,688

MOVING COIL MAGNETIC PEN RECORDER WITH COUPLED DRIVING AND VELOCITY COILS AND A MUTUAL INDUCTANCE COMPENSATING BUCKING SIGNAL COIL Original Filed Sept. 5, 1961 5 Sheets-Sheet 2 mxmg gw Array/M6) Oct. 25, 1966 A. D. BROWN, JR. ETAL 3,281,533

MOVING COIL MAGNETIC PEN RECORDER WITH COUPLED DRlVING AND VELOCITY COILS AND A MUTUAL INDUCTANCE COMPENSATING BUCKING SIGNAL COIL Original Filed Sept. 5, 1961 5 Sheets-$heet 5 United States Patent T MOVING COIL MAGNETIC PEN RECORDER WITH COUPLED DRIVING AND VELOCITY COILS AND A MUTUAL INDUC'IANCE COMPENSAT- ING BUCKING SIGNAL COIL Arling Dix Brown, Jr., Cleveland Heights, and Chester L. Morris, Richmond Heights, Ohio, assignors to Clevite Corporation, a corporation of Ohio Original application Sept. 5, 1961, Ser. No. 135,943, now Patent No. 3,088,788, dated May 7, 1963. Divided and this application Nov. 13, 1962, Ser. No. 236,869

3 Claims. (Cl. 324-125) This is a division of application Serial Number 135,943, filed September 5, 1961, now Patent 3,088,788, issued on May 7, 1963.

This invention is directed to improvements related to a moving coil magnetic pen recorder.

A principal aspect of the present invention is concerned with improving the structural rigidity of the moving coil in the pen recorder, so that the stresses to which it is subjected will not produce deformation of the coil which would detract from the linearity of the pen recorders response.

Another aspect of this invention is directed to correcting for errors arising from the mutual induction between the driving coil for the recording stylus and a velocity coil associated therewith to damp the movements of the driving coil. In accordance with this aspect of the invention, a stationary bucking coil is positioned in inductive relationship to the driving coil and is connected to substantially cancel out the effects of mutual induction between the driving coil and the velocity coil without, however, detracting from the desired damping effect of the velocity coil.

Further objects and advantages of the present invention will be apparent from the following detailed description of certain presently-preferred embodiments thereof, which are illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 is a section through a magnetic pen recorder in accordance with this invention, showing the moving coil and associated parts partly in section and partly in elevation;

FIGURE 2 is a section through the linkage for driving the recording stylus, taken along the line 22 in FIG- URE 1;

FIGURE 3 is a section taken along the line 33 in FIGURE 1;

FIGURE 4 is an exploded perspective view of the moving coil, the central core, and the core support in the pen motor of the FIGURE 1 recorder;

FIGURE 5 is a perspective view of the linkage for converting rotational movement of the moving coil to rectilinear movement of the recording tip of the stylus in the recorder of FIGURE 1;

FIGURE 6 is a schematic diagram of the electrical circuit which includes the pen recorder of FIGURE 1;

FIGURE 7 is a perspective view of the feedback transducer associated with the pen recorder of FIGURE 1; and

FIGURE 8 is a perspective view of the movable armature of this transducer.

Referring first to FIGURE 1, the present invention is shown as embodied in a magnetic pen recorder for making a visual record of electrical input signals. In broad out- 3,281,688 Patented Oct. 25, 1966 line the recorder comprises a pen motor including a permanent magnet system having opposite polarity pole pieces 30 and 31, a rotatably mounted driving coil 32 of electrically conductive wire between these pole pieces, and a shaft 33 connected to the coil to turn in unison with it, and a linkage system L driven by the pen motor shaft 33 and including a pen 34 for recording a substantially straight-line trace laterally across a moving record web 35.

The electrical input signals which are to be recorded are applied to the coil 32. In accordance with wellknown principles, the coil 32 turns about its axis from a neutral position through an angle which is substantially linearly proportional to the amplitude of the input signal. A restoring force, tending to return the driving coil to a position determined by the input signal, may be provided by a spring or by an electrical feedback signal which is impressed on the coil. In the preferred embodiment, this restoring force is provided by a feedback signal from an electromechanical transducer T operating in response to the moving coil.

Referring to FIGURES 1 and 3, the pole pieces 30 and 31 present spaced, confronting, concave, cylindrical pole faces 36 and 37 (FIG. 3) which are on a common circle about a central axis extending vertically in FIG- URE l. The pole pieces are of soft magnetic material. They have fiat-co-planar bottom faces 38 and 39 which engage the opposite pole tips 40 and 41 of a conventional horse-shoe permanent magnet 42. The pole pieces 30 and 31 are held spaced apart from one another by a rigid piece 43 (FIG. 3) of non-magnetic metal, such as brass or stainless steel.

A core 44 of soft magnetic material is fixedly mounted centrally in the gap between the opposite pole faces 36 and 37. A holder 45 (FIG. 4) of non-magnetic material, such as aluminum, engages the core in this position. As shown in FIGURE 4, this holder comprises an elongated leg 46 of arcuate cross-section, which is secured by screws 47 to the non-magnetic piece 43, and integral annular opposite end portions 48 and 49. The core 44 engages the inside face of the support leg 46 and is held in place by the screws 47. As best seen in FIGURE 3, the core presents arcuate surfaces 50 and 51 which are equally spaced from, and concentric with, the opposite pole faces 36 and 37, respectively. The core is formed with an axial through passage 52.

The moving coil 32 is made up of a multiplicity of turns of very fine, enameled, copper wire wound lengthwise around a generally rectangular coil frame 53 of nonmagnetic material, such as aluminum, and potted adhesively thereto into an integral unit. Throughout its length the coil frame 53 is channel-shaped in crosssection, as best seen in FIGURE 3. The unitary coil structure presents a first leg 54 extending lengthwise in the space between pole face 36 and core surface 50, an opposite leg 55 extending lengthwise in the space between pole face 37 and core surface 51, and opposite end members 56 and 57 joining these legs and extending across the opposite ends of the core 44 in spaced relation thereto, as shown in FIGURE 1. As shown in FIGURE 3, the base of the channel at each leg 54 and 55 of the coil structure has an arcuate configuration concentric with the respective pole face near it. The unitary coil structure throughout its entire extent is spaced from the pole faces 36 and 37 and from the core 44. The frame is provided primarily for convenience in winding the coil and may be omitted, if desired.

In accordance with an important aspect of the present invention the coil structure is physically reinforced against deformation due to torsional stresses and bending stresses in the plane of the coil structure. To this end there is provided a rigid shaft 33 extending freely through the central passage 52 in the core 44. The opposite end members 56, and 57 of the coil structure are fixedly secured to this shaft, preferably by an adhesive epoxy resin. As shown in FIGURE 1, a cross pin 59 extends through shaft 33 within the end member 56 of the coil structure in the gap formed by the wires on the frame. This pin assists in anchoring the coil structure to the shaft. Preferably, the shaft 33 is hollow.

It has been found that the presence of this one-piece shaft 33 extending lengthwise through the core 44 and attached to the opposite ends of the coil structure greatly enhances the structural rigidity of the coil assembly, so that the latter does not deform out of shape as a result of the stresses to which it is subjected. This is important to insure that undesired extraneous mechanical deflections will not occur to affect the stability of the overall feedback system of the pen recorder.

In FIGURE 1 an insulation plate 60 is bonded to the lower end of the coil assembly. A plurality of electrical terminal posts 61, which are connected to the coil, extend down through this plate. Insulated lead-in wires 62 connected to these posts extend through an opening 63 in the shaft 33 into the hollow interior of the shaft. These lead-in wires extend down and out the lower end of the shaft, where they are connected in the external electrical circuit.

The lower end portion 49 of the core support member 45 receives a flanged sleeve 64 (FIG. 1). A Belleville spring washer 65 is engaged between the top face of core support portion 49 and a flange 64a on the upper end of sleeve 64. A ball bearing assembly 66 is engaged between sleeve 64 and the coil shaft 33. The outer race member of this ball bearing assembly has a flange 67 at its upper end which overlies the upper end of sleeve 64. An annular collar 68, which is fixedly connected to shaft 33, is engaged between the upper end of the inner race member of the ball bearing assembly 66 and the lower face of insulation plate 60. Collar 68 has a side opening 69 which registers with the shaft opening 63 to pass the lead-in wires 62. With this arrangement, the spring washer 65 resiliently biases shaft 33 to a fixed axial position.

An insulation plug 70 closes the lower end of shaft 33. This plug has a passage 71 for passing the lead-in wires An insulation plate 72 overlies the upper end of coil 32. Directly above this plate a collar 73 is clamped rigidly to shaft 33. This collar carries the movable element 75 of the feedback transducer T, which will be described in detail hereinafter.

The upper end portion 48 on the core support member 45 has an internal annual groove 76, as best seen in FIG- URES 1 and 4. An annular cover plate 77 is bolted to the top of core support portion 48 and extends down into the groove 76 in the latter. A ball bearing assembly 78 is engaged between this cover plate and the shaft 33. The outer race member of this bearing has a lateral flange 79 on its lower end which engages beneath the cover plate 77. The collar 73 has a reduced diameter upper end extremity 80 which engages beneath the inner race member of this bearing.

A linkage, which is the subject of our copending application, Serial No. 135,943, filed September 5, 1961, now Patent No. 3,088,788, of which the present application is a division, is provided for converting the rotational movement of shaft 33 into substantially straightline movement of the point 34a of the pen 34 throughout a relatively long stroke.

This linkage includes a rigid lateral arm 81 fixedly connected to shaft 33 to turn in unison therewith. This arm extends perpendicular to the shaft in a direction away from the record web 35. At its free end the linkage arm 81 carries a post 82, which extends in spaced, parallel relationship to the shaft 33. The post 82 is rigidly secured to linkage arm 81 in any suitable manner. A hollow second shaft member 83 is mounted on post 82 for rotation about the latters axis. Shaft member 83 has a cylindrical periphery of a predetermined diameter d except at 84 where it is cut away (FIG. 2) to receive the free end of linkage arm 81. Above and below this arm the shaft member 83 presents annular upper and lower portions 85 and 86 (FIG. 1) which completely surround the post 82. At these portions, ball bearing sets 87 and 88 are engaged between the post 82 and shaft member 83.

An upwardly extending piece 89 is connected to the upper end of shaft member 83 at the side of the latter which is disposed toward the pen motor shaft 33, as best seen in FIGURE 5. The recording pen 34 is mounted in cantilever fashion on this piece 89, so that the pen moves in unison with shaft member 83. The recording pen extends to the opposite side of the pen motor shaft 33 from shaft member 83.

As shown in FIGURES 1 and 5, a hub 91 integral with cover plate 77 extends upward from the latter. This hub has a peripheral surface of a predetermined diameter a which is cylindrical about the axis of shaft 33. As best seen in FIGURES 2 and 5, this stationary hub is cut away at 92 to pass the linkage arm 81 and to permit angular movement of the latter about the axis of shaft 33 as shaft 33 turns.

A flexible band 93 (FIGS. 2 and 5) of suitable metal is fixedly attached at one point 94 to the cylindrical periphery of the stationary hub 91 and is fixedly attached at one point 95 on the opposite side of the cylindrical periphery of shaft member 83. The band 93 extends tautly (i.e., without slack) between hub 91 and shaft 83 on opposite sides of linkage arm 81.

When the pen motor shaft 33 rotates about its own fixedly-positioned axis, the coupling provided by band 93 between stationary hub 91 and shaft 83 causes the shaft 83 to turn about its own axis in a direction opposite to the direction in which shaft 33 turns. At the same time shaft 83 swings in an are about the axis of the pen motor shaft 33, due to the rigid linkage arm 81 connecting shaft 33 and the pivot post 82 for shaft 83.

For a better understanding of certain important aspects of the invention, reference is now made to the electrical circuit diagram shown schematically in FIGURE 6.

The moving coil 32, which is the drive coil of the pen motor, is shown as being mechanically coupled (dashed lines) to the linkage L for driving the rectilinear recording pen 34.

A velocity coil V is wound on the same coil frame 53 as the drive coil 32. The function of this coil is to sense the instantaneous velocity of the drive coil 32 and produce a feed-back signal for damping the movement of the drive coil.

The effects of mutual induction between the drive coil 32 and the velocity coil V produce an error in this feedback signal. In accordance with the present invention, this error may be substantially eliminated by providing a bucking coil B connected electrically in series opposition with the velocity coil. As shown in FIGURE 3, this bucking coil may be wound on an insulation frame which is bolted to the flat surface on the core 44. The bucking coil thus is positioned stationary, but in mutually inductive relationship to the drive coil 32.

The arrangement is such that the voltage induced in the bucking coil B is equal and opposite to the component of the velocity feedback voltage which is due to mutual induction between drive coil 32 and velocity coil V. These cancel each other, leaving as the corrected velocity feedback signal only the voltage due to the velocity of the drive coil 32 and velocity coil V.

As shown in FIGURE 6, the input signal which is to be recorded is applied first to an attenuator 131 and then to a differential amplifier 132 before being applied to the drive coil 32 of the pen motor.

A second input signal is applied to amplifier 132 from a summing and shaping network 134. One input to network 134 is the aforementioned corrected velocity feedback signal from velocity coil V. A second input to network 134 is a bias voltage from a suitable adjustable bias source 135. A third input to network 134 is a position voltage, whose magnitude depends upon the rotational position of the drive coil 32.

In the differential amplifier 132, the signal from summing and shaping network 134 is compared with the input signal from 131, and if there is a difference between these two signals, an amplified signal is applied to the drive coil to restore the pen to its correct position.

As already mentioned a feedback transducer T is associated with the pen recorder. The details of this transducer and of its feedback circuit are not the subject of the present invention, but they are described herein for the sake of completeness. Preferably, this transducer is of the general type disclosed in US. Patent 2,631,272 to Smith. Referring to FIGURE 7, the transducer comprises a laminated magnetic core 136 having opposite end legs 137 and 138, a back leg 139 interconnecting the end legs, an arcuate front leg 140 interconnecting the end legs, and a center leg 141 which extends from the back leg and terminates in an arcuate enlargement 142 extending in close-spaced, parallel relationship to the inside face of the front leg 140.

An input coil 143 is wound on the center leg 141 of the core. As shown in FIGURE 6, this input coil is connected to be energized by an oscillator 144.

A pair of series-connected output coils 145 and 146 are wound on the back leg of the core on opposite sides of the center leg. These output coils are connected in ,series opposition with each other across the input terminals of a phase sensitive detector 147 (FIG. 6). Oscillator 144 provides another input signal to the detector 147. The output of this detector 147, which is a voltage proportional to the position of the pen, is applied to the position voltage input terminal of the summing and shaping network 134.

The transducer T also includes a movable armature member 75 which is connected to the coil shaft 33, as already described. This armature member is a single loop of electrically conductive material and is made up of laterally spaced legs 148 and 149 (FIG. 8) which are bifurcated at their front ends to straddle the front leg 140 of the transducer core. A bridging segment 150 joined to these legs at their front ends is disposed in the air gap between the confronting faces of the core portions 142 and 140.

When the armature 75 is positioned in alignment with the axis of the center core leg 141, the flux induced in the center leg by the AC. voltage applied to coil 143 divides equally between the two end legs 137 and 138 of the core and produces equal and opposite voltages in the coils 145 and 146. Under this condition therefore, the net output voltage from the position transducer T is zero. This is the condition which is obtained when the drive coil 32 is in its centered position.

When the armature 75 is displaced away from this centered position, the flux divides unequally between the end legs 137 and 138 of the core. The amplitude of the resulting output voltage from transducer T varies linearly with the amount of angular displacement of the armature 75 from its centered position. In the phase sensitive detector 147, the transducer voltage is compared with the input voltage from oscillator 144 to determine the sign or polarity of the position voltage, depending upon the direction in which the armature has been moved from its centered position.

The position voltage output signal from the phase sensitive detector 147 has a magnitude which depends upon the angular displacement of the armature 75 from its centered position. Since armature 75 moves angularly in unison with the drive coil 32, the position voltage varies as a straight line function of the angular displacement of drive coil 32 from its neutral position.

The lateral displacement of the recording tip 34a from its zero position is almost a straight line function of the angular displacement of the drive coil 32, as described in detail in our aforementioned copending application, Serial No. 135,943, now Patent No. 3,088,788. However, its deviation from an exactly straight line relationship increases as the displacement increases. This error is compensated by proper design of the feedback transducer T and phase sensitive detector 147. Thus, as the magnitude of the position voltage increases the network 134 produces an output signal to the amplifier 132 which is a linear function of the pen movement. The net result is that the pen tip is caused to have a lateral displacement which is precisely linearly proportional to the input signal which is being recorded.

In the event that the record chart is passed over a straight edge and the recording stylus records on the paper at this edge, then the lateral displacement of the recording portion of the stylus would be a tangent function. In such event, the feedback arrangement would be designed to correct for the resulting non-linearity of the stylus movement laterally.

While a presently-preferred embodiment of this invention has been described in detail and illustrated in the accompanying drawings, it is to be understood that various modifications, omissions and refinements which depart from the disclosed embodiment may be adopted without departing from the spirit and scope of this invention.

We claim:

1. A transducer device comprising, a magnet system having a pair of stationary opposite pole pieces which present concave pole faces which face toward one another, a stationary core of magnetizable material between said pole faces, a driving coil extending in a loop lengthwise around said core and presenting a first lengthwise leg disposed between the core and one of said pole faces and a second lengthwise leg disposed between the core and the other of said pole faces, said driving coil throughout its extent being spaced from said core and from said pole faces, a rotatable rigid shaft extending lengthwise through said core and projecting beyond the opposite ends of the core, said shaft being connected rigidly to the driving coil structure beyond each end of the core and reinforcing the driving coil against stresses, a device coupled to said shaft to be moved in response to the turning of the shaft, a velocity coil rotatable with said driving coil to produce a velocity feedback signal, a bucking coil in inductive relationship to said driving coil for producing a bucking signal substantially equal to the component of said velocity feedback signal which is due to mutual induction between the driving coil and the velocity coil, means for subtracting said bucking signal from the velocity feedback signal to provide a corrected velocity feedback signal, and means, including lead-in wires connected to the driving coil, for applying electrical signals, including said corrected velocity feedback signal, to the driving coil.

2. The transducer device of claim 1 wherein said shaft is hollow and has an opening leading into its hollow interior beyond one end of the core, and said lead-in wires extend from the driving coil through said opening into the interior of the shaft.

3. A transducer device comprising, a magnet system having an air gap, a driving coil rotatable in said gap, a device driven by said driving coil, a velocity coil rotatable 7 8 with said driving coil to produce a velocity feedback 2,351,353 6/1944 McCarty 324-125 signal, a bucking coil in inductive relationship to sai 2,466,691 4/1949 Daniels 324-425 driving coil for producing a bucking signal substanti y 2,535,065 12/1950 Heiland 324154 equal to the component of said velocity feedback signal 5 0,257 7 1951 s 324 125 which is due to mutual induction between the driving coil 5 9 3 7 5 1 53 Bakke 324 125 and the velocity coil, means for subtracting said bucking 2 708 737 5/1955 Skidmore 324 125 signal from the velocity feedback signal to provide a cor- 2882496 4/1959 Greibach rected velocity feedback signal, and means for applying 3088788 5/1963 Brown et 5 346 139 electrical signals, including said corrected velocity feedbaCk Signal to the driving coil 10 WALTER L. CARLSON, Primary Examiner.

References Cited by the Examiner UNITED STATES PATENTS 799,733 9/1905 Hartmann' 324 155 AQUILINO, ROLINEQ I 2,351,079 6/ 1944 Strobel 324-425 15 AsslstantExammers.

LEYLAND M. MARTIN, Examiner. 

3. A TRANSDUCER DEVICE COMPRISING, A MAGNET SYSTEM HAVING AN AIR GAP, A DRIVING COIL ROTATABLE IN SAID GAP, A DEVICE DRIVEN BY SAID DRIVING COIL, A VELOCITY COIL ROTATABLE WITH SAID DRIVING COIL TO PRODUCE A VELOCITY FEEDBACK SIGNAL, A BUCKING COIL IN INDUCTIVE RELATIONSHIP TO SAID DRIVING COIL FOR PRODUCING A BUCKING SIGNAL SUBSTANTIALLY EQUAL TO THE COMPONENT OF SAID VELOVITY FEEDBACK SIGNAL WHICH IS DUE TO MUTUAL INDUCTION BETWEEN THE DRIVING COIL AND THE VELOCITY COIL, MEANS FOR SUBTRACTING SAID BUCKING SIGNAL FROM THE VELOCITY FEEDBACK SIGNAL TO PROVIDE A CORRECTED VELOCITY FEEDBACK SIGNAL, AND MEANS FOR APPLYING ELECTRICAL SIGNALS, INCLUDING SAID CORRECTED VELOCITY FEEDBACK SIGNAL, TO THE DRIVING COIL. 